Niobium—phosphorus coupling constants

Journal of Organometalfic Chemistry, 479 (1994) 207-211

Niobium-phosphorus

coupling constants

Vernon C. Gibson, Roberto Gobetto *, Robin K. Harris, Christina Langdale-Brown and Ulrich Siemeling Department

of Chemistry,

University of Durham,

South Road, Durham DHI 3LE (UK)

(Received December 1, 1993)

Abstract Magic-angle spinning “P NMR spectra of solid niobium half-sandwich compounds are shown to yield values of (93Nb, 31P) coupling constants not accessible from solution-state spectra, and also to provide more accurate values of chemical shifts. Results are reported and discussed for eight compounds and the general value of NMR spectra of solids for these and similar cases is emphasized. Key words: Niobium; Phosphorus; Coupling; Nuclear magnetic resonance; Magic-angle spinning

1. Introduction

been

207

208

V.C. Gibson et al. / Nb-P coupling constants

TABLE

1. Solid-state

NMR data for the half-sandwich

Compound

niobium complexes

a

Wppm

1JNbP

la lb lc 2a

CpNb(NMe)CI,(PMe,) CpNb(NMe)Cl,(PMePh,) CpNb(NMe)Cl,(PPh,) CpNb(N’Bu)Cl,(PMe,)

1.7 12.3 32.1 5.0

490 476 446 508

2b 3 4 5 6 7

CpNb(NAr)Cl,(PMe,) Cp*Nb(NMe)CI,(PMes) Cp*Nb(NArXCOXPMe,) Cp*Nb(NArXPhC = CPhXPMe,)

7.7 3.1 10.1 11.7 ca. -5 ca. -16

471 523 913 696 ca. 260 ca. 240

Cp*Nb(NArXH),(PMe,) Cp*NbCl,(CO),(PMe,)

a Ar = 2,6-Pr$,Hs;

Oxidation state

1/Hz

V V V V

172 1,2 192 2

V V III III V III

172 2 3 3 3 8

Cp’ = CsMe,.

eliminated by MAS, though they are averaged to a certain extent. Such effects have recently been reviewed 161. Fortunately, in the case of g3Nb it would appear that, for the magnetic field used in the present work (7.4 T), the controlling ratio, K = I ,~&4!32!3 - Ov, I (where xNb and vNb are the quadrupole coupling constant and Zeeman frequency of niobium respectively),

PR3 = PMe3 (la) PMephz (lb)

I’% (1~)

is sufficiently low that perturbation theory can be applied to the problem. Then, although the splittings in the 31P spectra become unequal, their average remains precisely equal to the (Nb, P) coupling constant in magnitude. Strictly speaking, this has only been proved when the quadrupolar and indirect (J) coupling tensors are axially symmetrical and are both coaxial with the Nb,P internuclear vector, but significant errors in

ArN&Nb
R’ = Bu’ (2a) k

(2b)

PMe3 (3)

* cl

III..

Cl’ b

ArN

\

-WI

\

H

H

PMe3

m

.a\\ co

t .co PMe3 (7)

Ar = 2,6-i-F&&3 Fig. 1. Molecular

Synthesis reference

structures of the compounds

examined.

KC. Gibson et al. / h&P

measurement of J are not expected if there are departures from these criteria. We have illustrated [71 the application of the spectral analysis methods outlined above for several complexes of transition metals involving phosphine ligands, including one compound of niobium, so the technique itself will not be described in detail here.

couplingconstants

209

a

2. Results and discussion The ten compounds examined are listed in Table 1, with the structures illustrated in Fig. 1. Seven of them involve niobium in its highest (+ V) oxidation state, whereas three are in oxidation state +III. Solutionstate 31P NMR spectra are difficult to obtain and are relatively uninformative. A typical example is illustrated in Fig. 2a, which shows that no splittings aris$rg from (Nb, P) coupling are visible, and even the P chemical shift cannot be obtained with any accuracy, since the resonance band is cu. 70 ppm in width at half height. By contrast, the 31P MAS spectrum of the solid (Fig. 2b) contains relatively sharp lines clearly forming a simple decet with apparently equal spacings. Of the compounds examined, the one (4) referred to in Fig. 2 has the lowest quadrupole coupling constant, therefore giving the sharpest lines, with the most equal spacings. In consequence, the 31P chemical shift and the value of I J,, I are the most accurately measured. In contrast, the spectra of two’ compounds (6 and 7) contain an anomalous distribution of linewidths for which no convincing explanation is yet available, and consequently the NMR parameters are least well-defined. A more typical 31P spectrum is that for compound lb, illustrated in Fig. 3, where the modest nature of both linewidths and splitting asymmetry can be seen. The asymmetry is a consequence of the transferred secondorder quadrupolar effects mentioned in the Introduction. The splitting patterns were analysed using the perturbation theory approach outlined in ref. [61 and used in ref. [7], though it should be noted that certain assumptions are made which are unlikely to be strictly valid for the cases discussed here (this is not expected to affect the reported derived parameters significantly). The results for the 31P chemical shifts and (93Nb, 31P) coupling constants are given in Table 1. Both are clearly sensitive to the chemical structure of the compounds. Replacement of methyl groups by phenyl in molecules of type 1 causes deshielding, as expected, and also leads to a small but significant decrease in I J,, I. A wider variation in both shifts and couplings is apparent for the remaining systems. The niobiumphosphorus coupling constants range from ca. 240 to 913 Hz, which shows this parameter to have potential

I”~‘I~“‘I~““““I’~“I””

300

200

100

0

-100

-200

-300

&pm Fig. 2. Phosphorus-31 NMR spectra of Cp’Nb(NArXCOXPMe3X4): (a) In solution in C,D,, at 100 MHz; (b) in the solid state, at 121.4 MHz. The ten central lines in (b) represent isotropic transitions. The remaining lines are spinning sidebands.

value for structural correlations. The formal oxidation state of niobium is not a dominant influence - indeed, the smallest and largest values of I JNbP I occur for two of the compounds of O.S. III. Clearly more work is needed to establish useful relations to structure, but there is little doubt that I J,, I can be chemically useful, nor that this requires NMR spectra of solids. Spin-spin coupling from other quadrupolar metals to phosphorus can be evaluated similarly, and Table 2 gives a few examples (not an exhaustive list) for comparison. Obviously there are substantial variations, but all the values are hundreds of Hz (ranging up to nearly 2 kHz in the case of copper) and are chemically significant, underlining the value of solid-state (MAS) NMR as compared to more-traditional solution-state work. However, in some cases (see, for example, ref. [17]), splittings arising from (metal, phosphorus) coupling can be observed in solution-state spectra, both of 31P and of the metal. 3. Experimental

section

3.1. Nuclear magnetic resonance

Phosphorus-31 NMR spectra were obtained at 121.4 MHz using a Varian VXR 300 spectrometer at ambi-

210

6,

KC. Gibson et al. / Nb-P coupling constants

100 I

50

6

I

-50I

-100 I

Fig. 3. 121.4 MHz 31P CPMAS spectrum of CpNb(NMe)Clz(PMePhzXlb): The ten central lines are isotropic transitions, the remaining peaks being spinning sidebands. The unequal spacings of the decet arise from transferred second-order quadrupolar effects [7]. The intensity variations are produced by the interplay of dipolar, indirect coupling and shielding tensors [7]. Spectrometer conditions: contact time 2.5 ms; recycle delay 1.0 s; number of transients 600, spinning rate 4.82 kHz.

ent probe temperature under conditions of ‘H -+ 31P cross-polarization, high-power proton decoupling and magic-angle spinning. Contact times varied from 1 to 8

TABLE 2. Some metal-phosphorus

ms, the recycle delay was 1 s (except in the case of 6 where it was 2 s), and between 500 and 3000 transients were acquired for each spectrum. A Doty 7 mm o.d.

coupling constants obtained by MAS NMR

Compound

Coupling

I Jim I/Hz

Literature reference

Me,P-AlCl,/zeolite Mn,(CO),PPh, Mn,(CO)s(PPh,), (OCjdBrMnPPh,R b (OC),MnPPh,(CH,),S0, ’ Co,(CO),,PPh, Co,(CO),(PPh,), HFeCo,(CO),,PPh, Co~~CO),(C,H,XPPh,), (Ph3P),CuN03 [CU,(‘BUHPCH,PH’BU),]~+~BF~ Cu(PPh,),X d,e Cu(PPhz)-X d*e [L,CuXj,;dJ Cp(OC),MoPPh,(CH,),

27Al,31P 55Mn, 31P

9 7 7 10 10 7 7 7 7 11 12 13 14 15 10

aN-trans-Ru(PEt3)2(CO)2(C = CPh), (~-H)Ru,(CO),,(CL-PPh2)[CL~-PPh(C6H4)1 g

99Ru, 31P

300 297 335 = 197 to 210 206 to 214 425 324 a 472 522 = 1450 956 1160 to 1440 890 to 980 1358 to 1865 190 (m = 3) 157 (m = 4) 104 129 and 159

59Co, 31P

63cu, 31P

9s/97M0, 3tP

16 16

a Average for different 31P sites; b R = Et, Pr, Bu, Pe; ’ n = 2,3,4; d X = halide. e There are a number of other papers by Healy and co-workers giving (Cu, P) coupling constants derived from solid-state MAS 31P NMR. f L = 1-phenyl-3,4-dimethylphosphole or 1-phenyldibenzophosphole. ‘Data for the PPh, group (two nonequivalent sites).

KC. Gibson et al. / Nb-P coupling constants

MAS probe was employed with spinning rates between 4.8 and 5.3 kHz. Phosphorus chemical shifts are quoted relative to the signal from aqueous (85%) phosphoric acid, using the high-frequency-positive convention. They were actually measured using a sample of brushite (6, = 1.1 ppm) by replacement. The second-order quadrupolar effects were analysed according to ref. [7] to yield isotropic chemical shifts and coupling constants. The compounds are air- and moisture-sensitive, so it was necessary to use airtight inserts in the MAS rotors, as described previously [18]. The solution-state 31P spectrum shown in Fig. 2a was obtained using a Bruker AC250 spectrometer. Carbon-13 CPMAS spectra at 75.43 MHz were also obtained for all the compounds. They served to confirm the molecular structures but were otherwise unremarkable. 3.2. Half-sandwich niobium imido complexes Preparations of compounds la, 2a,b and 3 have been reported [2]. The syntheses of compounds 4-7 have been outlined in preliminary reports [3,5,8] and full accounts will be published elsewhere. Compounds lb and lc were prepared by a procedure analogous to that used for la [1,2]. Acknowledgments

R.G. thanks the Italian CNR for a fellowship and the Research and Initiatives Committee of the University of Durham for financial support. U.S. thanks the Deutsche Forschungsgemeinschaft for a postdoctoral fellowship. We are grateful to the U.K. Science and Engineering Research Council for access to the Solidstate NMR Service based at the University of Durham and to Dr. D.C. Apperley for expert assistance in obtaining the spectra.

211

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